The invention relates to a softstart valve means, comprising a main valve member placed between a primary duct conducting pressure medium at a primary pressure and a secondary duct, said main valve member being biased, or able to be biased by a closing force into a closed position disconnecting the connection between the primary duct and the secondary duct, said main valve member being able to be switched over by an oppositely acting opening force overriding the closing force into an open position opening the connection between the primary duct and the secondary duct, the opening force being derived in a secondary pressure controlled operational mode of the valve means by the gradually increasing secondary pressure of the pressure medium, such pressure medium flowing, bypassing the main valve member, by way of a choke means from the primary duct into the secondary duct.
Such a softstart valve means designed for secondary pressure controlled operation is described in the textbook “Pneumatische Steuerungen”, by Werner Deppert and Kurt Stoll, published by Vogel Verlag, 10th edition, 1994 pages 160 and 161. The main valve member of a shut off valve in the form of a two-position valve is in this case placed on the connection between a primary duct connected with a pressure source and a secondary duct connected with the loads to be supplied and selectively can shut off (closed position) or open (open position) the connection. Normally the main valve member is in this case biased into the closed position by a closing force based on the primary pressure.
Following the switching on of the valve means the pressure medium, flowing by way of primary duct, may bypass the closed main valve by way of a first choke means into the secondary duct, it flowing by way of a choke means into the secondary duct. The secondary pressure obtaining in the secondary duct is returned for producing a servo or setting force acting on the main valve and switches the main valve into the open position, when the secondary pressure (and the said setting force) has reached a predetermined opening force. Thus it is possible to ensure that the softstart valve means only permits unchoked passage of fluid, when the secondary pressure has risen to a certain pressure level. Accordingly all in all a relatively slow build up of pressure is achieved in the connected loads and the occurrence of pressure surges is prevented, which could lead to damage or accidental rapid movements of the connected loads.
There are however applications in which using a softstart valve controlled in a manner dependent on the secondary pressure only an insufficient operational mode is possible. If for example on the secondary side a relatively high rate of fluid consumption occurs, it may in certain circumstances last a long time before the secondary pressure necessary for producing the opening pressure is built up on the secondary side, something which may considerably delay the putting plant into operation. Under unfavorable conditions there may be no switching of the main valve member into the open position at all.
In order to deal with such unsatisfactory condition purely time controlled or timed softstart valves have being become known, in which the main valve member is switched gradually and in a manner independent of the currently obtaining secondary pressure after a predetermined period of time into the open position. As an example for such a softstart valve reference may be had to the product catalog “Der Pneumatik-Katalog”, issue of 1997/1998 of FESTO AG & Co, page 9.1/42-1 with a description of a so-called pressure build up valve. Since in the case of such a purely time controlled valve no interrogation of the secondary pressure and accordingly of the pressure obtaining at the connected loads takes place, there is however the risk, in the case of improper use, of a premature opening of the main valve member being entailed, possibly with resulting damage owing to an excessive pressure surge.
In many cases account could be taken of the respective application by having an appropriate selection of the type of softstart valve utilized. In the case of changing operational conditions this has however turned out to be impractical. As a rule there is no realistic possibility of modification of a plant to suit different applications.